(i) Field of the Invention
The present invention relates to a novel surface treating solution which can give excellent anticorrosion to zinc-plated steel plates which are bare or have been coated, and these steel plates are, for example, electrically zinc-plated steel plates, electrically zinc alloy-plated steel plates, melted zinc-plated steel plates, alloyed and melted zinc-plated steel plates or vapor-depositionally zinc-plated steel plates. Furthermore, the present invention relates to a surface treating process for zinc-plated steel plates.
(ii) Description of the Prior Art
It is known that various chromate treatments and metal flash treatments of nickel, magnesium and the like have been heretofore used in order to chemically improve the anticorrosion properties of zinc-plated steel plates. Known examples of the above-mentioned chromate treatments include a reaction type chromate treatment, a coating type chromate treatment and an electrolysis type chromate treatment. Furthermore, a known example of the metal flash treatment is the electrolytic deposition treatment of metallic nickel from an acidic nickel plating solution, as described in, for example, Japanese Patent Laid-open No. 56-55592. These treatments are mainly used as undercoating treatments.
However, in the above-mentioned chromate treatments, hexavalent chromium is used as an essential component to obtain good performance. The employment of hexavalent chromium, which is harmful and difficult of disposal, leads to an environmental problem.
When the final treatment for the zinc-plated steel plates is carried out by the above-mentioned metal flash treatment using a metal such as copper, nickel or cobalt which is nobler than zinc, the corrosion of zinc is inversely accelerated unpreferably by a local polarization function with the zinc material.
Therefore, the conventional examples can provide good anticorrosion properties but involve the environmental problem of a waste solution owing to the employment of hexavalent chromium. In addition, it is known that a metal such as aluminum or magnesium contributes to the improvement of anticorrosion properties of zinc, but the process for depositing the metal on the zinc-plated steel plates from an aqueous solution has some problems to be solved.